1. Field of the Invention
The present invention relates to a piezoelectric transformer, a piezoelectric transformer drive circuit and a piezoelectric transformer drive method used for various high-voltage generation apparatuses.
Furthermore, the present invention relates to a cold cathode tube drive apparatus using a piezoelectric transformer used for various high-voltage generation apparatuses, more particularly to a cold cathode tube drive apparatus using a piezoelectric transformer having sensor electrodes provided independently of primary and secondary electrodes.
2. Related Art of the Invention
FIG. B18 shows the structure of a Rosen-type piezoelectric transformer, a typical structure of a conventional piezoelectric transformer. This piezoelectric transformer has the advantages that it can be made more compact than an electromagnetic transformer, is noncombustible and does not cause noise due to electromagnetic induction.
The portion designated by 1001 is the low impedance portion of the piezoelectric transformer and used as an input portion in the case when the transformer is used for voltage step-up. The low impedance portion 1001 is polarized in a thickness direction A. Primary electrodes 1003U and 1003D are disposed on the main faces of the low impedance portion in the direction the thickness thereof. On the other hand, the portion designated by 1002 is a high impedance portion and used as an output portion in the case when the transformer is used for voltage step-up. The high impedance portion 1002 is polarized in the longitudinal direction B. A secondary electrode 1004 is disposed at the end face in the longitudinal direction.
FIG. B19, detailed later, is a graph showing the characteristic of the above-mentioned piezoelectric transformer. When the load of the piezoelectric transformer is infinite (indicated by a curve P1 in FIG. B19), it is possible to obtain a very high step-up ratio in the case when the drive frequency of the piezoelectric transformer is equal to the resonance frequency thereof. On the other hand, when the load becomes small (indicated by a curve P2 in FIG. B19), the step-up ratio lowers. Because of this characteristic, the piezoelectric transformer has been used as the power sources for cold cathode tubes in recent years. A cold cathode tube drive apparatus using a piezoelectric transformer can efficiently generate a high voltage. However, since it can easily generate a high voltage, if the piezoelectric transformer is controlled improperly, an overvoltage may generate from the piezoelectric transformer, resulting in the breakdown of the piezoelectric transformer and the like. To prevent this kind of breakdown and the like, it is proposed to provide an overvoltage protection circuit for the cold cathode tube drive apparatus.
FIG. B20 is a block diagram showing the configuration of a cold cathode tube drive apparatus using a conventional piezoelectric transformer. In FIG. B20, numeral 1193 designates a variable oscillation circuit generating an AC drive signal for driving a piezoelectric transformer 1200. The output of the variable oscillation circuit 1193 is usually a pulse waveform signal. The high-frequency components of the pulse waveform signal is eliminated by a waveform shaping circuit 1191, whereby the pulse waveform signal is converted into an AC signal close to a sine wave signal. The output of the waveform shaping circuit 1191 is voltage-amplified to a level enough to drive the piezoelectric transformer 1200 by a drive circuit 1192 and input to the primary electrode (indicated by 1003U in FIG. B18) of the piezoelectric transformer 1200. The output voltage stepped up by the piezoelectric effect of the piezoelectric transformer 1200 is taken out from its secondary electrode (indicated by 1004 in FIG. B18).
The high voltage output from the secondary electrode is applied to a series circuit comprising a cold cathode tube 1197 and a feedback resistor 1198 and to an overvoltage protection circuit portion 1190. In the over voltage protection portion 1190, a voltage divider circuit comprising voltage division resistors 1199a and 1199b divides the high voltage output from the secondary electrode of the piezoelectric transformer 1200. A comparison circuit 1195 compares the voltage divided by the voltage divider circuit with a set value Vref1 and generates an error voltage. The error voltage output from the comparison circuit 1195 is applied to an oscillation control circuit 1194. The oscillation control circuit 1194 controls the variable oscillation circuit 1193 so that the high voltage output from the secondary electrode of the piezoelectric transformer 1200 is equal to Vref1xc3x97(electric resistance value of the resistor 1199a+electric resistance value of the resistor 1199b)/electric resistance value of the resistor 1199a. The oscillation control circuit 1194 does not accept the output from the overvoltage protection circuit 1190 while the cold cathode tube 1197 is lit.
Furthermore, the voltage (current detection value) generated across the feedback resistor 1198 by the current flowing through the series circuit comprising the cold cathode tube 1197 and the feedback resistor 1198 is applied to a comparison circuit 1196. The comparison circuit 1196 compares the current detection value with a set value Vref2 and outputs an error voltage. The error voltage output from the comparison circuit 1196 is applied to the oscillation control circuit 1194. The variable oscillation circuit 1193 is controlled by the oscillation control circuit 1194 so that a nearly constant current flows through the cold cathode tube 1197.
As described above, the oscillation control circuit 1194 operates on the basis of the output from the comparison circuit 1195 before the lighting start of the cold cathode tube 1197, and the oscillation control circuit 1194 operates on the basis of the output from the comparison circuit 1196 while the cold cathode tube 1197 is lit.
In this way, the cold cathode tube 1197 is lit stably. Even if the resonance frequency is changed depending on the change in the load of the piezoelectric transformer, ambient temperature and the like, the drive frequency can follow the resonance frequency automatically by driving the cold cathode tube 1197 using the above-mentioned drive apparatus.
Next, the operation of this drive apparatus will be described referring to FIG. B19. FIG. B19 is a graph showing the operation characteristic of the piezoelectric transformer. As clearly shown in FIG. B19, the step-up ratio has the maximum value at the resonance frequency according to the operation characteristic of the piezoelectric transformer. Usually, drive control is carry out by using a frequency higher than the resonance frequency of the piezoelectric transformer.
When driving the piezoelectric transformer, its drive frequency is set at a frequency (fa) higher than the resonance frequency at the time of start. When the voltage divided by the voltage division resistors 1199a and 1199b is smaller than the set voltage Vref1, the drive frequency is lowered close to the resonance frequency by the oscillation control circuit 1194 and the variable oscillation circuit 1193. When the drive frequency is close to the resonance frequency, the step-up ratio of the piezoelectric transformer increases, and its output voltage rises. When the output voltage reaches the lighting start voltage (Vb) of the cold cathode tube 1197, the cold cathode tube 1197 is lit. As a result, the load of the piezoelectric transformer lowers from an infinite value to about several hundred kxcexa9. Therefore, the operation characteristic of the piezoelectric transformer shifts from the curve P1 to curve P2.
Accordingly, the operation of the oscillation control circuit 1194 is shifted from the operation depending on the output of the comparison circuit 1195 to the operation depending on the output of the comparison circuit 1196. Furthermore, the output of the piezoelectric transformer shifts from Vb to Va although the frequency fb remains the same. If the current detection value generated by the feedback resistor 1198 is smaller than the set value Vref2 at this time, the drive frequency is lowered until the current detection value reaches the set value, whereby the step-up ratio of the piezoelectric transformer is raised thereby to increase the current flowing through the cold cathode tube 1197. On the other hand, if the current detection value generated by the feedback resistor 1198 is larger than the set value Vref2, the drive frequency is raised, whereby the step-up ratio of the piezoelectric transformer is lowered thereby to decrease the current flowing through the cold cathode tube 1197. In this way, the piezoelectric transformer is controlled so that the current detection value generated by the feedback resistor 1198 is equal to the set value Vref2.
If the cold cathode tube 1197 is not lit even when the output voltage reaches the lighting start voltage (Vb), in other words, if the current detection value generated by the feedback resistor 1198 remains zero even when the voltage value obtained by dividing the output voltage of the piezoelectric transformer 1200 by the voltage division resistors 1199a and 1199b reaches the set value Vref1, the overvoltage protection circuit 1190 stops frequency sweep at the variable oscillation circuit 1193 via the oscillation control circuit 1194. This prevents the piezoelectric transformer 1200 from breaking, and also prevents an overvoltage from generating from the piezoelectric transformer 1200.
The current flowing through the cold cathode tube is controlled and the piezoelectric transformer is protected against overvoltages by configuring the cold cathode tube drive apparatus using the piezoelectric transformer as described above.
In the above-mentioned conventional piezoelectric transformer, the step-up ratio differs greatly depending on whether the cold cathode tube 1197 is at the time of start (non-lighting) or at the time of stable operation (lighting). Since the step-up ratio at the time of start of the cold cathode tube 1197 is far larger than that at the time of stable operation, the transformer can easily output a high voltage. In order to use the high voltage, the overvoltage protection circuit 1190 is configured in parallel with the series circuit comprising the cold cathode tube 1197 and the feedback resistance 1198, wherein overvoltage protection is carried out by feeding back a voltage proportional to the output voltage from the voltage division resistors 1199a and 1199b connected to the secondary electrode of the piezoelectric transformer 1200.
However, in this kind of conventional overvoltage protection circuit, a high voltage is divided and the voltage obtained by the division is fed back. Therefore, if the resistance values of the voltage division resistors 1199a and 1199b constituting the voltage divider circuit are lowered, the load of the piezoelectric transformer 1200 decreases, whereby the step-up ratio required to light the cold cathode tube 1197 cannot be obtained. Furthermore, a current is consumed unnecessarily by the voltage division resistors 1199a and 1199b. Because of these problems, the resistance values of the voltage division resistors 1199a and 1199b are required to be sufficiently large. As a result, the detection voltage cannot be obtained accurately because of variations in resistance values, parasitic capacitances to a PC board and the like, whereby the overvoltage protection circuit may malfunction.
Furthermore, if the voltage required to light the cold cathode tube 1197 increases, the voltages for feedback applied to the voltage division resistors 1199a and 1199b also increase greatly. Accordingly, a sufficient creepage distance must be provided for a PC board to conform to safety standards, thereby causing a problem of making the circuit larger.
In addition, Japanese Laid-open Patent Application No. Hei 9-9640 discloses a drive apparatus wherein a current IL flowing through a load RL is detected by a current detection means 1168, the result of this detection is compared with a brightness set voltage V1, an error voltage obtained as the result of the comparison is filtered and phase-compensated by an integrator 1162, voltage/frequency conversion is performed by a V-F converter 1163, and a piezoelectric transformer 1161 is driven by a drive means 1167 to control the current flowing through the load RL as shown in FIG. B17. This drive apparatus is configured so that a surge clamper 1169 is connected to the output of the piezoelectric transformer 1161 in parallel with the load RL to prevent the overload protection circuit from malfunctioning. However, even in this case, the output from the secondary high voltage portion is fed back. Therefore, it is necessary to route high-voltage lines on the PC board. As a result, this configuration causes problems of possible malfunctions owing to stray capacitances, insufficient creepage distances and the like.
Furthermore, Japanese Laid-open Patent Application No. Hei 11-68185 has proposed a configuration wherein a part of the primary multilayer portion of a piezoelectric transformer is used as a feedback electrode. However, this feedback electrode is used to simplify the drive circuit. Therefore, this is insufficient as a countermeasure for the overvoltage protection for the piezoelectric transformer.
In view of the problems encountered in the above-mentioned conventional piezoelectric transformers, an object of the present invention is to provide a piezoelectric transformer, a piezoelectric transformer drive circuit, a piezoelectric transformer drive method and a cold cathode tube drive apparatus using piezoelectric transformer capable of carrying out overvoltage protection at a voltage lower than a value used conventionally.
The 1st invention of the present invention is a piezoelectric transformer comprising:
a piezoelectric substrate mainly formed of a piezoelectric material,
primary electrodes which are formed on said piezoelectric substrate and to which a voltage is applied,
a secondary electrode which is formed on said piezoelectric substrate and from which a voltage higher than the voltage applied to said primary electrode is output, and
a third electrode which is formed on said piezoelectric substrate and from which a voltage lower than the voltage output from said secondary electrode is output.
The 14th invention of the present invention is a piezoelectric transformer drive circuit comprising:
a piezoelectric transformer for outputting a voltage input to a primary terminal from a secondary terminal by virtue of a piezoelectric effect, said piezoelectric transformer having a sensor electrode for detecting a voltage lower than the output voltage from said secondary terminal,
a drive circuit for driving said piezoelectric transformer,
a variable oscillation circuit for supplying a desired frequency and a desired voltage from said drive circuit to said piezoelectric transformer,
a discharge tube, the input terminal of which receives the output voltage of said piezoelectric transformer and the output terminal of which is connected to a feedback resistor,
an overvoltage protection circuit for detecting the output voltage from said sensor electrode, for comparing said output voltage with a first reference voltage and for outputting the result of the comparison,
comparison means for comparing the voltage value of said feedback resistor with a second reference voltage so that the current flowing through said discharge tube becomes constant and for outputting the result of the comparison,
a frequency control circuit for controlling the drive frequency of said piezoelectric transformer on the basis of the result of the comparison from said overvoltage protection circuit before the lighting start of said discharge tube or for controlling the drive frequency of said piezoelectric transformer on the basis of the result of the comparison from said comparison means while said discharge tube is lit.
The 15th invention of the present invention is a piezoelectric transformer drive method for outputting a voltage input to a primary terminal from a secondary terminal by virtue of a piezoelectric effect, wherein:
a voltage is detected from a third electrode which is provided on said piezoelectric transformer to output a voltage lower than the output voltage of said secondary terminal, and
the result of said detection is used for overvoltage protection for the output voltage of said secondary terminal of said piezoelectric transformer.
As described above, the piezoelectric transformer of the present invention is, for example, a piezoelectric transformer, provided with a sensor electrode as an example of the third electrode in a part of the piezoelectric transformer, carries out protection against opening at a relatively low output voltage and also carries out feedback by using the output from the sensor electrode.
The 16th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer comprising:
a piezoelectric transformer for stepping up the voltage input from a primary electrode by a piezoelectric effect, for outputting the stepped-up voltage from a secondary electrode and for outputting a detection voltage in proportion to the output voltage from a sensor electrode,
a piezoelectric transformer drive portion for generating an AC voltage, the frequency of which is variable, for amplifying said AC voltage to a predetermined level and for supplying the amplified voltage to said piezoelectric transformer,
a cold cathode tube driven by the output voltage from said secondary electrode of said piezoelectric transformer,
a resistor for detecting the current flowing through said cold cathode tube as a voltage,
an oscillation control circuit for controlling the frequency of said AC voltage output from said piezoelectric transformer drive portion on the basis of said voltage detected by said resistor so that the current flowing through said cold cathode tube becomes a predetermined value,
an overvoltage protection circuit for controlling the frequency of said AC voltage output from said piezoelectric transformer drive portion on the basis of said detection voltage from said sensor electrode via said oscillation control circuit before the lighting start of said cold cathode tube and for stopping the frequency control of said AC voltage output from said piezoelectric transformer drive portion in the case when said detection voltage from said sensor electrode exceeds a predetermined value.
The 17th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 16th invention, wherein said piezoelectric transformer has said primary electrodes and said sensor electrodes disposed opposite to each other so as to form a polarized structure in the direction of the thickness of a piezoelectric element, has said secondary electrode disposed so as to form a polarized structure in the longitudinal direction of said piezoelectric element, and steps up the input voltage applied to said primary electrode to obtain an output voltage from said secondary electrode and to obtain a detection voltage in proportion to said output voltage from said sensor electrode.
The 18th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 16th invention, wherein said piezoelectric transformer is characterized in that in a first region in the longitudinal direction of a piezoelectric element, a first electrode is disposed on one of the surfaces in the direction of the thickness, a second electrode and a third electrode are disposed in sequence from said first electrode in said direction of the thickness inside said piezoelectric element, a fourth electrode is disposed on the other surface of said piezoelectric element opposite to said surface so that said electrodes are disposed opposite to each other at predetermined distances and close to one of the end faces of said piezoelectric element in the longitudinal direction thereof, and a fifth electrode is disposed on the other end face opposite to said end face, and also characterized in that in a second region in the longitudinal direction of said piezoelectric element, a polarization structure is formed in the longitudinal direction of said piezoelectric element, said first and second electrodes are used as said primarily electrodes, said third and fourth electrodes are used as said sensor electrodes, said fifth electrode is used as said secondary electrode, and the input voltage applied to said primary electrode is step up to obtain an output voltage from said secondary electrode and to obtain a detection voltage in proportion to said output voltage from said sensor electrode.
The 19th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 16th invention, wherein said piezoelectric transformer drive portion comprises:
an electromagnetic transformer having a primary winding to which said DC power source is supplied and a secondary winding connected to said primary electrode of said piezoelectric transformer, for stepping up said AC voltage and for supplying the stepped-up voltage to said piezoelectric transformer, and
a switching circuit for controlling the frequency of said AC voltage supplied to said piezoelectric transformer by switching said DC voltage supplied to said primary winding of said electromagnetic transformer.
The 20th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 17th invention, wherein said piezoelectric transformer drive portion comprises:
an electromagnetic transformer having a primary winding to which said DC power source is supplied and a secondary winding connected to said primary electrode of said piezoelectric transformer, for stepping up said AC voltage and for supplying the stepped-up voltage to said piezoelectric transformer, and
a switching circuit for controlling the frequency of said AC voltage supplied to said piezoelectric transformer by switching said DC voltage supplied to said primary winding of said electromagnetic transformer.
The 21st invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 18th invention, wherein said piezoelectric transformer drive portion comprises:
an electromagnetic transformer having a primary winding to which said DC power source is supplied and a secondary winding connected to said primary electrode of said piezoelectric transformer, for stepping up said AC voltage and for supplying the stepped-up voltage to said piezoelectric transformer, and
a switching circuit for controlling the frequency of said AC voltage supplied to said piezoelectric transformer by switching said DC voltage supplied to said primary winding of said electromagnetic transformer.
The 22nd invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 19th invention, wherein said electromagnetic transformer is formed of first and second electromagnetic transformers, said switching circuit is provided with first and second switching transistors connected to the primary windings of said first and second electromagnetic transformers respectively, and said first and second electromagnetic transformers are used in series or parallel to drive said piezoelectric transformer.
The 23rd invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 20th invention, wherein said electromagnetic transformer is formed of first and second electromagnetic transformers, said switching circuit is provided with first and second switching transistors connected to the primary windings of said first and second electromagnetic transformers respectively, and said first and second electromagnetic transformers are used in series or parallel to drive said piezoelectric transformer.
The 24th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 21st invention, wherein said electromagnetic transformer is formed of first and second electromagnetic transformers, said switching circuit is provided with first and second switching transistors connected to the primary windings of said first and second electromagnetic transformers respectively, and said first and second electromagnetic transformers are used in series or parallel to drive said piezoelectric transformer.
The 25th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 22nd invention, wherein the AC voltage supplied from one of said first and second electromagnetic transformers to said piezoelectric transformer is used as a reference voltage, and said piezoelectric transformer is driven on the basis of the difference value between said detection voltage from said sensor electrode and said reference voltage.
The 26th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 23rd invention, wherein the AC voltage supplied from one of said first and second electromagnetic transformers to said piezoelectric transformer is used as a reference voltage, and said piezoelectric transformer is driven on the basis of the difference value between said detection voltage from said sensor electrode and said reference voltage.
The 27th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 24th invention, wherein the AC voltage supplied from one of said first and second electromagnetic transformers to said piezoelectric transformer is used as a reference voltage, and said piezoelectric transformer is driven on the basis of the difference value between said detection voltage from said sensor electrode and said reference voltage.
The 28th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with anyone of the 17th, 20th and 23rd inventions, wherein said piezoelectric transformer is driven in the primary mode of vertical vibration in the longitudinal direction by an AC voltage signal, the half-wave length of which is equal to the length of said piezoelectric transformer in the longitudinal direction thereof.
The 29th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with anyone of the 18th, 21st and 24th inventions, wherein said piezoelectric transformer is driven in the primary mode of vertical vibration in the longitudinal direction by an AC voltage signal, the half-wave length of which is equal to the length of said piezoelectric transformer in the longitudinal direction thereof.
The 30th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with any one of the 17th to 20th inventions, wherein said piezoelectric transformer is driven in the secondary mode of vertical vibration in the longitudinal direction by an AC voltage signal, the one wavelength of which is equal to the length of said piezoelectric transformer in the longitudinal direction thereof.
The 31st invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with any one of the 16th to 18th inventions, wherein said oscillation control circuit is provided with a switching device for selectively controlling the frequency of said AC voltage output from said variable oscillation circuit on the basis of said detection voltage from said sensor electrode before the lighting start of said cold cathode tube or on the basis of the detection voltage by said resistor after the lighting start of said cold cathode tube.
The 32nd invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with any one of the 16th to 20th, 22nd and 23rd inventions, wherein a voltage divider circuit comprising resistors is connected to said sensor electrode of said piezoelectric transformer, and the output of said voltage divider circuit is used as said detection voltage from said sensor electrode.
The 33rd invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 28th invention, wherein a voltage divider circuit comprising resistors is connected to said sensor electrode of said piezoelectric transformer, and the output of said voltage divider circuit is used as said detection voltage from said sensor electrode.
The 34th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 29th invention, wherein a voltage divider circuit comprising resistors is connected to said sensor electrode of said piezoelectric transformer, and the output of said voltage divider circuit is used as said detection voltage from said sensor electrode.
The 35th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 30th invention, wherein a voltage divider circuit comprising resistors is connected to said sensor electrode of said piezoelectric transformer, and the output of said voltage divider circuit is used as said detection voltage from said sensor electrode.
The 36th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 31st invention, wherein a voltage divider circuit comprising resistors is connected to said sensor electrode of said piezoelectric transformer, and the output of said voltage divider circuit is used as said detection voltage from said sensor electrode.
The 37th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with any one of the 16th to 18th inventions, wherein the load connected to said sensor electrode is determined so that the relationship between the output capacitance of said piezoelectric transformer and said load connected to said secondary electrode is equal to the relationship between the capacitance between said sensor electrodes disposed opposite to each other and said load connected to said sensor electrode.
The 38th invention of the present invention is a cold cathode tube drive apparatus using a piezoelectric transformer in accordance with the 37th invention, wherein said load connected to said sensor electrode has at least double the resistance value of the impedance calculated by 1/(2xc3x97xcfx80xc3x97fdxc3x97Cs), wherein the capacitance between the pair of said sensor electrodes is Cs and the resonance frequency of said piezoelectric transformer is fd.
With the above-mentioned configuration, it is possible to attain a compact, highly efficient, highly reliable drive apparatus by not routing high-voltage lines to the protection circuit and by preventing malfunctions due to unnecessary vibration of the piezoelectric transformer at the time of protecting the piezoelectric transformer against overvoltages.